Method for producing rare earth sintered magnets

ABSTRACT

A method for producing rare earth sintered magnets includes the steps of pressing and compacting an alloy powder for the rare earth sintered magnets, thereby preparing a plurality of green compacts, arranging the green compacts on a receiving plane in a direction in which a projection area of each of the green compacts onto the receiving plane is not maximized, and heating the green compacts, thereby sintering the green compacts and obtaining a plurality of sintered bodies.

TECHNICAL FIELD

[0001] The present invention relates to a method for producing rareearth sintered magnets.

BACKGROUND ART

[0002] Rare earth sintered magnets currently used extensively in variousfields of applications include a samarium-cobalt (Sm—Co) type magnet anda neodymium-iron-boron type magnet (which will be herein referred to asan “R-T-(M)-B type magnet”). Among other things, the R-T-(M)-B typemagnet (where R is at least one of the rare earth elements includingyttrium (Y) and is typically neodymium (Nd), T is either Fe alone or amixture of Fe, Co and/or Ni, M is at least one additive selected fromthe group consisting of Al, Ti, Cu, V, Cr, Ni, Ga, Zr, Nb, Mo, In, Sn,Hf, Ta and W, and B is either boron alone or a mixture of boron andcarbon), is used more and more often in various types of electronicappliances. This is because the R-T-(M)-B type magnet exhibits a maximumenergy product (BH)_(max) that is higher than any of various other typesof magnets, and yet is relatively inexpensive.

[0003] A rare earth sintered magnet is produced by pulverizing a rareearth alloy into an alloy powder, pressing and compacting the alloypowder under a magnetic field to obtain a green compact (as-pressedcompact) and then sintering the green compact in a sintering furnace. Ifthe rare earth element such as neodymium to be included in the R-T-(M)-Btype magnet is oxidized during the sintering process, the resultantmagnetic properties deteriorate significantly. Thus, to avoid thedisadvantageous oxidation, the atmosphere inside the sintering furnaceis normally a vacuum or a reduced-pressure inert atmosphere of Ar, He,or any other inert gas. In sintering multiple green compacts, thosegreen compacts are loaded into a hermetically sealable sintering case(which is also called a “sintering pack”) and then the sintering case,including those green compacts, is heated in its entirety to increasethe productivity. Also, when a great number of green compacts are to besintered simultaneously, a sintering case, equipped with a number ofsintering base plates piled up like shelves, is used. In that case, theas-pressed green compacts are arranged on the sintering base plates andthen those plates are stored like shelves inside the sintering case.

[0004] For example, green compacts 95 to be processed into sinteredmagnets for a motor are sintered after having been arranged inside asintering case 9 as shown in FIGS. 3A and 3B.

[0005] In the example illustrated in FIGS. 3A and 3B, the sintering case9 includes a bottom container 90 and a cover 92 to be fitted with thebottom container 90. The bottom container 90 includes a bottom plate 90a and a sidewall 90 b. Inside the bottom container 90, a number ofsintering base plates 94 are vertically piled up with a predeterminedgap provided between them by spacers 96. The sintering case 9 is heatedup to an elevated temperature of about 1,000° C. or more, for example,during the sintering process. Accordingly, the bottom container 90 andthe cover 92 are both made of a material with high thermal resistance(e.g., molybdenum or SUS310).

[0006] The sidewall 90 b of the bottom container 90 surrounds theperiphery of the sintering base plates 94 and supports the cover 92thereon at the upper edge thereof. The space surrounded with thesidewall 90 b (i.e., storage space) has a horizontal dimension (i.e., awidth) that is slightly greater than the width of the sintering baseplates 94. The difference may be on the order of several millimeters toseveral centimeters. In any case, this sintering case 9 is designed soas to have a narrow gap between the sintering base plates 94 and thesidewall 90 b. This narrow gap is adopted to store the greatest possiblenumber of green compacts 95 inside the sintering case 9 simultaneouslyas efficiently as possible. This is because the narrower the gap, thegreater the width of the sintering base plates 94 can be. In addition,when the gap between the sintering base plates 94 and the sidewall 90 bis small, even if the sintering case 9 is vibrated during itstransportation, for example, the sintering base plates 94 cannot moveinside the sintering case 9 so much as to collapse the spacers 96 on thesintering base plates 94 unintentionally.

[0007] As shown in FIGS. 4A through 4C, each of the green compacts 95has curved surfaces including a concave surface 95 a and a convexsurface 95 b. When the green compact 95 shown in FIG. 4A is viewed alonga plane that crosses the concave and convex surfaces 95 a and 95 b atright angles, the cross section of the compact 95 has a shape includingtwo arcs. For example, the concave and convex surfaces 95 a and 95 b mayconstitute respective portions of two cylindrical surfaces havingmutually different radii of curvature. In that case, the outer radiusdefined by the convex surface 95 b may be greater than the inner radiusdefined by the concave surface 95 a. A green compact having such a shapeis called a “curved green compact” or an “arched green compact”. Asshown in FIG. 4A, this green compact 95 includes two curved surfaces(i.e., the concave and convex surfaces 95 a and 95 b, which will beherein also referred to as “principal surfaces”) that are opposed toeach other; two side surfaces 95 d that are opposed to each other withthe two curved surfaces 95 a and 95 b interposed between them; and twoend surfaces 95 c that cross both the curved surfaces 95 a and 95 b andthe side surfaces 95 d substantially at right angles. The principalsurfaces 95 a and 95 b are greater in area than any of the othersurfaces of the green compact 95. Typically, the end surfaces (orbottoms) 95 c are smaller in area than any other surface of the greencompact 95.

[0008] The green compacts 95 of such a shape are mounted on each of thesintering base plates 94 so as not to contact with each other, e.g., sothat the horizontal edges of the concave surface 95 a or the center ofthe convex surface 95 b is in contact with the sintering base plate 94as shown in FIGS. 4B and 4C. These arrangements are used to prevent thegreen compacts 95 from turning over in the manufacturing and processingstep of mounting the green compacts 95 on the sintering base plate 94 orloading the sintering base plate 94 into the case 9, for example. Forthat purpose, the green compacts 95 are arranged to have their center ofmass located at the lowest possible level (i.e., so that their top islocated at the lowest possible level) when mounted on the sintering baseplate 94. To increase the degree of orientation, the green compacts 95(e.g., green compacts to be processed into R-T-(M)-B type magnets, inparticular) have a green density that is lower than that of greencompacts to be processed into ferrite magnets. The green compacts 95 tobe processed into R-T-(M)-B magnets may have a green density of about3.9 g/cm³ to about 5.0 g/cm³, for example. Accordingly, these greencompacts 95 are very brittle and easily crack or chip on impact withsomething hard (e.g., the instant they fall or are dropped). Thus, thesegreen compacts 95 should be arranged so as not to turn over so easily.It should be noted that the green compacts 95 arranged on the samesintering base plate 94 may have been either subjected to the compactionprocess individually or obtained by cutting and dividing a single greencompact into multiple smaller bodies.

[0009] Furthermore, if the green compacts 95 that have been mounteddirectly on the sintering base plate 94 are sintered, then the resultantsintered bodies 95 and the sintering base plate 94 may sometimes bepartially fused together unintentionally. This is because the rare earthelement such as Nd included in the R-T-(M)-B type alloy powder and ametal element included in the sintering base plate 94 may cause aeutectic reaction at a temperature that is equal to or lower than thesintering temperature. If the base plate 94 and the sintered bodies 95are partially fused together, the size of the green compacts 95 beingsintered does not decrease smoothly with the sintering process, thuspossibly cracking or chipping the resultant sintered bodies 95. Also,even if the base plate 94 and the sintered bodies 95 are not fusedtogether, non-uniform friction may be created between the base plate 94and the sintered bodies 95, thus also possibly cracking the sinteredbodies 95 on their surface that is in contact with the sintering baseplate 94.

[0010] Thus, to prevent the sintering base plate 94 and the sinteredbodies 95 from being fused together, the surface of the sintering baseplate 94 is coated with a bedding powder (not shown) according to aknown technique so that the green compacts 95 can be sintered on thebedding powder (see, for example, Japanese Laid-Open Publication No.4-154903). The bedding powder needs to be a powder of a material thatexhibits low reactivity with the green compacts 95 and high chemicalstability at an elevated temperature. When the green compacts 95 includea rare earth metal, the bedding powder may be a powder of a materialexhibiting low reactivity with the rare earth metal, e.g., a powder of arare earth oxide such as neodymium oxide or yttrium oxide. By using sucha bedding powder, the sintering base plate 94 and the sintered bodies 95are not fused together, and therefore, portions of the sintered bodies95 that are in contact with the base plate 94 are neither damaged (e.g.,cracked) nor deformed.

[0011] However, if multiple green compacts 95 are arranged inside thesintering case 9 as shown in FIGS. 3A and 3B, then the number of greencompacts 95 that can be stored inside the sintering case 9 at the sametime is relatively small, and the sintering process cannot be performedso efficiently. Specifically, when the flat-plate green compacts 95 aremounted so as to have their center of mass located at the lowestpossible level, the projection area of each of those green compacts 95on the base plate 94 is rather great, thus decreasing the number ofgreen compacts 95 that can be arranged within a limited area. As usedherein, the “projection area” of each green compact 95 means the areathat is covered by the green compact 95 on the base plate 94.

[0012] Also, if the green compacts 95 are mounted as shown in FIG. 4B or4C, each of these green compacts 95 is in contact with the base plate 94in just a narrow area. Then, as the sintering process advances, the(frictional) stress that is created due to the shrinkage of the greencompact 95 will be concentrated on the contact portions. In that case,even if the bedding powder is used as described above, the sintered body95 is still damaged or deformed often by the frictional stress that iscreated.

[0013] Furthermore, when the green compact 95 is mounted as shown inFIG. 4C, portions located around the center of the convex surface 95 bof the compact 95 are damaged or deformed. Thus, it is impossible toremove only the damaged or deformed portion of the sintered body 95 anduse the remaining portion thereof. On the other hand, when the greencompact 95 is mounted as shown in FIG. 4B, the concave surface 95 a ofthe compact 95 has its horizontal edges deformed. This concave surface95 a has a shape that should not be deformed to fit the resultantsintered magnet on the rotor shaft of a motor. Accordingly, it is alsodifficult to remove only the deformed portions therefrom and process theremaining portion into a predetermined shape for a sintered magnet. Thatis to say, if any of the sintered bodies that have been mounted as shownin FIG. 4B or 4C becomes defective, then the defective sintered bodycannot be used anymore, thus decreasing the yield of sintered magnetssignificantly.

[0014] On the other hand, Japanese Laid-Open Publication No. 61-125114discloses a technique of reducing the number of defective (e.g., warpedor deformed) sintered bodies in making relatively thin rare earthsintered magnets. According to the technique disclosed in JapaneseLaid-Open Publication No. 61-125114, a green compact having a smallthickness is sandwiched between a pair of thicker green compacts that ismade of the same material, and has the same shape, as the former greencompact. Also, according to the technique, a powder of a material thatdoes not react with the green compacts easily is interposed betweenthese green compacts and/or between the green compact and the base platewhen needed.

[0015] In the method disclosed in Japanese Laid-Open Publication No.61-125114, however, not only the thin green compact but also two otherthicker green compacts should be prepared to obtain a single sinteredbody of the desired small thickness, thus decreasing the yield of therare earth alloy powder material. Also, according to such a technique,it is difficult to increase the number of green compacts 95 that can beloaded into the sintering case 9 at the same time. Furthermore, insintering the green compacts 95 having a shape such as that shown inFIG. 4A, it is difficult to sufficiently reduce the damage ordeformation of the resultant sintered bodies 95 due to the frictionalstress created by the shrinkage of the green compacts 95 being sintered.It is rather understandable that the frictional stress, which is createdbetween the lowest one of the green compacts stacked and the base plate,would be increased to further damage or deform the resultant sinteredbody, because the total mass of the vertically stacked green compacts isapplied to the lowest green compact that is in contact with the baseplate.

[0016] As described above, the green compact of a rare earth alloypowder has a great specific gravity (e.g., a green compact of anR-T-(M)-B type alloy powder has a specific gravity of about 3.9 g/cm³ ormore) and is very brittle. Accordingly, when a frictional stress iscreated due to the shrinkage of the green compact being sintered (whichloses as much as about 40% or more of its volume), the sintered body iseasily damaged or deformed. Particularly when a green compact is mountedso as to have its center of mass located at a low level and to have asmall area of contact with the base plate as shown in FIG. 4B or 4C, theresultant sintered body is damaged or deformed very easily. In addition,it is also difficult to store such green compacts efficiently inside asintering case.

DISCLOSURE OF INVENTION

[0017] In order to overcome the problems described above, preferredembodiments of the present invention provide a method for producing rareearth sintered magnets that minimizes the number of damaged or deformedsintered bodies and greatly increases productivity.

[0018] A preferred embodiment of the present invention provides a methodfor producing rare earth sintered magnets. The method preferablyincludes the steps of pressing and compacting an alloy powder for therare earth sintered magnets, thereby preparing a plurality of greencompacts, arranging the green compacts on a receiving plane in adirection in which a projection area of each of the green compacts ontothe receiving plane is not maximized, and heating the green compacts,thereby sintering the green compacts and obtaining a plurality ofsintered bodies.

[0019] In one preferred embodiment of the present invention, the step ofarranging the green compacts preferably includes the step of arrangingthe green compacts on the receiving plane in a direction in which theprojection area of each of the green compacts onto the receiving planeis minimized.

[0020] In another preferred embodiment of the present invention, thestep of pressing and compacting an alloy powder preferably includes thestep of preparing a plurality of green compacts each having at least onecurved surface, and the step of arranging the green compacts preferablyincludes the step of arranging the green compacts on the receiving planeso that the at least one curved surface of each of the green compactscrosses the receiving plane substantially at right angles.

[0021] In still another preferred embodiment, the step (a) preferablyincludes the step of preparing a plurality of green compacts eachhaving: two principal surfaces that are opposed to each other; two sidesurfaces that are opposed to each other with the two principal surfacesinterposed therebetween; and two end surfaces that cross both theprincipal surfaces and the side surfaces substantially at right angles.The step (b) preferably includes the step of arranging the greencompacts on the receiving plane so that one of the two end surfaces ofeach of the green compacts contacts with the receiving plane.

[0022] In yet another preferred embodiment, the step of pressing andcompacting an alloy powder preferably includes the step of pressing andcompacting the alloy powder under an aligning magnetic field, and thestep of arranging the green compacts preferably includes the step ofarranging the green compacts on the receiving plane so that orientationdirections of the alloy powder are substantially parallel to thereceiving plane.

[0023] In yet another preferred embodiment, the step of pressing andcompacting an alloy powder preferably includes the step of preparing thegreen compacts having a green density of about 4.1 g/cm³ to about 4.5g/cm³.

[0024] In yet another preferred embodiment, the step arranging the greencompacts preferably includes the step of arranging the green compacts onthe receiving plane so that the green compacts are in contact with eachother in a horizontal direction (which is typically substantiallyparallel to the thickness direction of the green compacts).

[0025] In this particular preferred embodiment, the step of arrangingthe green compacts preferably includes the step of arranging the greencompacts, which have already been magnetized, on the receiving plane sothat the green compacts attract each other via a magnetic force producedbetween the green compacts.

[0026] Alternatively or additionally, the step of arranging the greencompacts may include the step of applying an anti-fusing agent to atleast portions of the green compacts and arranging the green compacts onthe receiving plane so that the green compacts come into contact witheach other via the anti-fusing agent. Typically, the anti-fusing agentis applied to a portion of each of the green compacts.

[0027] Specifically, the anti-fusing agent preferably includes a powderof Y₂O₃. More specifically, the Y₂O₃ powder preferably has a meanparticle size of about 1 μm to about 10 μm, more preferably about 3 μmto about 5 μm.

[0028] In this particular preferred embodiment, the step of arrangingthe green compacts preferably includes the step of applying slurry, inwhich the Y₂O₃ powder is dispersed in an organic solvent, to theportions of the green compacts.

[0029] In yet another preferred embodiment, the method may furtherinclude the step of removing a portion of each of the sintered bodies,which portion has been in contact with the receiving plane, and asurrounding portion thereof.

[0030] Another preferred embodiment of the present invention provides asintered magnet for use in a motor. The magnet is preferably produced bythe method according to any of the preferred embodiments of the presentinvention described above.

[0031] Other features, elements, characteristics, steps and advantagesof the present invention will become more apparent from the followingdetailed description of preferred embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0032]FIGS. 1A and 1B are respectively a cross-sectional view and a planview schematically illustrating how green compacts 95 may be arranged ina sintering process step of a method for producing rare earth sinteredmagnets according to a preferred embodiment of the present invention.

[0033]FIG. 2 is a perspective view illustrating how adjacent ones of thegreen compacts 95 shown in FIGS. 1A and 1B may be arranged.

[0034]FIGS. 3A and 3B are respectively a cross-sectional view and a planview schematically illustrating a known arrangement of the greencompacts 95 in a sintering process step of a conventional method forproducing rare earth sintered magnets.

[0035]FIGS. 4A, 4B and 4C illustrate what problems are caused by theknown arrangement of the green compacts 95 in the sintering processstep, wherein FIG. 4A is a perspective view of the green compact 95 tobe processed into a sintered magnet for a motor, and FIGS. 4B and 4C arecross-sectional views schematically illustrating how the green compact95 may be mounted on a base plate 94.

BEST MODE FOR CARRYING OUT THE INVENTION

[0036] Hereinafter, preferred embodiments of the present invention willbe described as being applied to a method for producing R-T-(M)-B typesintered magnets for use in a motor, for example. It should be noted,however, that the present invention is not limited to the followingspecific preferred embodiments but is broadly applicable to a method forproducing rare earth sintered magnets of any of various other types.

[0037] A method for producing rare earth sintered magnets according tovarious preferred embodiments of the present invention is mainlycharacterized by the manufacturing and processing step of sinteringgreen compacts. Accordingly, the following description of preferredembodiments of the present invention will be focused on this sinteringprocess step and the description of other manufacturing and processingsteps, which may be carried out by known techniques, will be omittedherein.

[0038] A method for producing rare earth sintered magnets according to apreferred embodiment of the present invention preferably includes thesteps of pressing and compacting an alloy powder for the rare earthsintered magnets, thereby preparing a plurality of green compacts,arranging the green compacts on a receiving plane in a direction inwhich a projection area of each of the green compacts onto the receivingplane is not maximized, and heating the green compacts, therebysintering the green compacts and obtaining a plurality of sinteredbodies.

[0039] In the step of arranging the green compacts, the green compactsare preferably stored in a case having the receiving plane. Thesintering step preferably includes the step of heating the case,including the green compacts therein, in its entirety. When such asintering case is used, the atmosphere for the sintering process stepcan be more uniform, for example.

[0040] In this method, the step of arranging the green compacts may becarried out by using the sintering case 9 shown in FIGS. 3A and 3B, forexample. In the drawings to be referred to in the following description,each member having substantially the same function as the counterpartshown in FIGS. 3A, 3B, 4A, 4B or 4C will be identified by the samereference numeral and the description thereof will be omitted herein.

[0041] In one preferred embodiment of the present invention, the greencompacts 95 to be processed into sintered magnets for a motor may bearranged as shown in FIGS. 1A, 1B and 2.

[0042] In this preferred embodiment, the green compacts 95 may have anouter diameter of about 22.13 mm, a width of about 26.14 mm, a thicknessof about 9.73 mm and a height of about 45 mm in the arrangement shown inFIGS. 1A and 1B, for example. In the sintering case 9, the bottom plate(i.e., the flat-plate portion) 90 a of the bottom container 90 thereofmay have approximate dimensions of 270 mm×305 mm×1 mm (thickness), whilethe cover 92 thereof may have approximate outer dimensions of 280 mm×315mm×70 mm (height) and a thickness of about 1.5 mm, for example. Thebottom container 90 and the cover 92 may be made of a material that canresist the heat generated in the sintering and other process steps,e.g., stainless steel or a refractory metal such as molybdenum. Forexample, the sintering case 9 may be made of SUS310. In that case, thecase 9 is not deformed due to the heat so much as a case 9 made ofSUS304.

[0043] In the preferred embodiment illustrated in FIGS. 1A and 1B, thegreen compacts 95 are arranged on the sintering base plate 94 that hasbeen mounted on the bottom plate 90 a of the bottom container 90.Alternatively, the sintering base plate 94 may be omitted and the greencompacts 95 may be mounted directly on the bottom plate 90 a of thebottom container 90. That is to say, either the surface of the baseplate 94 or that of the bottom plate 90 a functions as a surface thatreceives the green compacts 95 thereon. The sintering base plate 94 ispreferably used because a large number of green compacts 95 can bearranged thereon easily. The sintering base plate 94 may haveapproximate dimensions of 250 mm×300 mm×1 mm (thickness), for example.The sintering base plate 94 is preferably made of molybdenum. This isbecause molybdenum has low reactivity with the green compacts andexhibits good thermal conductivity and thermal resistance. The receivingplane of this sintering base plate 94 preferably has an average surfaceroughness Ra of about 1 μm to about 50 μm.

[0044] In this preferred embodiment, the green compacts 95 are arrangedon the base plate 94 in a direction in which the projection area of eachof the green compacts 95 onto the base plate 94 is minimized, unlike thearrangement shown in FIGS. 4A and 4B. According to such an arrangement,a greater number of green compacts 95 can be arranged within the samelimited area. Naturally, it is most efficient to arrange the greencompacts 95 in the direction in which the projection area of each greencompact 95 onto the base plate 94 is minimized. However, the greencompacts 95 may also be arranged in any other direction as long as theprojection area of each green compact 95 onto the base plate 94 is notmaximized. This is because unless the projection area is maximized, theprojection area decreases to a certain degree and the green compacts 95can be stored in the case 9 more efficiently. If the green compacts 95having a substantially flat-plate shape are arranged so as to have theircenter of mass located at a low level as in the prior art, then theprojection area of each green compact 95 onto the base plate 94 ismaximized as described above. In contrast, in this preferred embodimentof the present invention, the green compacts 95 are arranged so as tohave minimized projection areas.

[0045] When these green compacts 95 have curved surfaces (i.e., theconcave surface 95 a and convex surface 95 b) like the green compacts 95to be processed into sintered magnets for use in a motor, the greencompacts 95 are preferably arranged so that their curved surface(s) 95 aand/or 95 b cross(es) the surface of the base plate 94 substantially atright angles. As shown in FIGS. 4A, 4B and 4C, the green compact 95 hasa substantially flat-plate shape and the curved surfaces 95 a and 95 bas its opposed principal surfaces. If such a green compact 95 is mountedon the base plate 94 so that the curved surface 95 a or 95 b thereof isopposed to the surface of the base plate 94, then the area of contactbetween the green compact 95 and the base plate 94 is small, a greaterfrictional stress is created due to the shrinkage of the green compact95 being sintered and the resultant sintered body is damaged or deformedto a greater degree as already described with reference to FIGS. 4B and4C. In contrast, if the green compact 95 is mounted on the base plate 94so that a flat surface thereof (e.g., the bottom 95 c) is in contactwith the surface of the base plate 94 as shown in FIGS. 1A, 1B and 2,then the area of contact between the green compact 95 and the base plate94 increases and a smaller frictional stress is created due to theshrinkage of the green compact 95 being sintered. Furthermore, theamount of maximum shrinkage in the contact area between the greencompact 95 and the base plate 94 (i.e., the maximum value ofone-dimensional lengths decreasing) is smaller than the arrangementshown in FIGS. 3A and 3B. Thus, the frictional stress decreases becauseof this reason also. To prevent the green compact 95 and the base plate94 from being fused together unintentionally, a bedding powder ispreferably interposed between the green compact 95 and the base plate94.

[0046] However, if the green compact 95 is mounted on the base plate 94with its bottom 95 c facedown as shown in FIG. 2, then the green compact95 will have a center of mass located at a higher level and will falldown more easily. Also, in that case, it is much more troublesome toarrange a great number of green compacts 95 in such a position. Thequasi-flat-plate green compact 95 illustrated in FIG. 2 falls downparticularly easily. This is because its center of mass easily shiftsfrom its bottom 95 c even when the green compact 95 leans only slightly.Accordingly, where the green compacts 95 should be mounted on the baseplate 94 with their bottom 95 c facedown, the green compacts 95 arepreferably arranged thereon so as to come into contact with each otherhorizontally (typically, in a direction that is substantially parallelto the surface of the base plate 94 and substantially parallel to thethickness direction of the green compacts 95).

[0047] Particularly if the green compacts 95 have been magnetized (e.g.,if the green compacts 95 have acquired remanent magnetization during thecompaction process carried out under a magnetic field), then the greencompacts 95 attract each other via a magnetic force produced betweenthem. As a result, the green compacts 95 can be arranged in a rowstably. The green compacts 95 to be processed into sintered magnets foruse in a motor acquire remanent magnetization M while being subjected toa compaction process under an aligning magnetic field, and attract eachother via the remanent magnetization M as shown in FIG. 2. The magnitudeof the remanent magnetization M (i.e., remanence) is preferably fromabout 0.002 T to about 0.006 T. The green compact, which has beenobtained by compacting a material alloy powder under an aligningmagnetic field to make an anisotropic sintered magnet as describedabove, is preferably demagnetized incompletely so as to retain a certaindegree of remanent magnetization.

[0048] Also, as shown in FIG. 2, the direction of the remanentmagnetization M (which will be herein also referred to as the“orientation direction” of the green compacts or alloy powder) ispreferably substantially parallel to the direction in which the greencompacts 95 are arranged, i.e., a substantially horizontal direction(typically, substantially parallel to the surface of the base plate 94).The green compacts 95 exhibit anisotropic magnetic properties.Accordingly, the green compacts 95 being sintered shrink at a relativelyhigh rate in a direction that is substantially parallel to themagnetization direction thereof. For that reason, to minimize the amountof shrinkage, which is obtained by multiplying the shrinkage rate by thelength, the green compacts 95 have preferably been magnetized in adirection defined by the shortest one of the three dimensions of thegreen compacts 95. For example, the quasi-flat-plate green compacts 95have preferably been magnetized in the thickness direction as shown inFIG. 2. However, the direction of the remanent magnetization M is notlimited to the thickness direction but may be any other direction aslong as the green compacts 95 can be magnetized in such a manner as toattract each other. For instance, if the green compacts 95 to bearranged as shown in FIG. 2 have been magnetized in the height direction(i.e., vertically), then the green compacts 95 may be arranged in such amanner as to alternate their magnetization directions in the directionin which the green compacts 95 are arranged, i.e., so that themagnetization direction of one of the green compacts 95 is opposite tothat of a horizontally adjacent one of them. Then, the green compacts 95can also attract each other.

[0049] On the other hand, if there is no need to apply any aligningmagnetic field during the process step of preparing the green compacts(e.g., in making green compacts for isotropic magnets), a magnetic fieldmay also be applied afterward to the as-pressed, green compacts 95 sothat the green compacts 95 will have a remanence falling within therange specified above.

[0050] It should be noted that to arrange the green compacts 95 asstably as possible, the number of green compacts 95 that make up eachrow is preferably determined appropriately in accordance with the shapeof the green compacts 95 to be obtained. More specifically, to increasethe stability of the row of green compacts 95, the number of greencompacts 95 to be in contact with each other needs to be large enough toprevent the center of mass of the row of green compacts 95 from shiftingfrom the bottom of the row so easily even if the row is vibrated orleaned to an expected degree.

[0051] If the green compacts 95 are arranged so as to be adjacent toeach other, then the green compacts 95 might be fused togetherunintentionally during the sintering process. To avoid this unwantedsituation, an anti-fusing agent is preferably applied to at least thoseportions where the green compacts 95 are in contact with each other.That is to say, the green compacts 95 are preferably in contact witheach other with the anti-fusing agent interposed between them.

[0052] Just like the conventional bedding powder, the anti-fusing agentis also preferably made of a material exhibiting low reactivity with thegreen compacts 95, e.g., a rare earth oxide. Among other things, theanti-fusing agent preferably includes a powder of Y₂O₃. This is becauseY₂O₃ exhibits high chemical stability and is hardly reduced while thegreen compacts of a rare earth alloy powder are sintered. The Y₂O₃powder preferably has a mean particle size of about 1 μm to about 10 μm,and more preferably about 3 μm to about 5 μm.

[0053] The anti-fusing agent may be applied to the predeterminedportions of the green compacts 95 by coating those portions with slurryin which the anti-fusing agent (e.g., a powder of Y₂O₃) is dispersed inan organic solvent. The organic solvent is preferably a solvent having ahigh degree of volatility, e.g., a hydrocarbon based solvent such asisoparaffin or a lower alcohol based solvent such as ethanol. When apowder of Y₂O₃ is used as the anti-fusing agent, slurry in which theY₂O₃ powder is dispersed at a concentration of about 20 g/l inisoparaffin may be applied with a brush or a spray. If the slurry hassuch a concentration, the unwanted fusing can be prevented sufficientlyby applying the slurry to those portions just once with a brush. Ifnecessary, the slurry may have its concentration changed (e.g., within arange of about 10 g/l to about 800 g/l) or applied a greater number oftimes.

[0054] Optionally, the green compacts 95 may be immersed in the slurry.However, this technique is not preferable because a lot of organicsolvent must be absorbed into the green compacts 95 to increase theamount of carbon that will remain in the resultant sintered bodies. Forthat reason, the anti-fusing agent is preferably applied selectively tothe predetermined portions of the green compacts 95 by brushing thoseportions over, for example. In addition, when the easily volatilizableslurry having a concentration falling within the above-specified rangeis used, no drying process step has to be performed.

[0055] For example, in the conventional arrangement shown in FIGS. 3Aand 3B, only 100 green compacts 95 can be mounted on four base plates 94(i.e., 25 green compacts 95 per base plate) inside the sintering case 9that accommodates the base plates 94 having approximate dimensions of300 mm×260 mm. In contrast, according to the arrangement shown in FIGS.1A and 1B, as many as 130 green compacts 95 can be mounted on a singlebase plate 94. In the arrangement shown in FIGS. 1A and 1B, the gapbetween two adjacent rows of green compacts 95 is preferably about 10 mmor more and the gap between the inner walls of the sintering case 9 andthe green compact rows is preferably about 20 mm or more. These gaps areleft to allow the worker to mount the green compacts 95 on the baseplate 94 easily enough, and may be changed if necessary.

[0056] According to the arrangement of this preferred embodiment, thegreen compacts 95 can be arranged inside the sintering case 9 much moreefficiently than the conventional arrangement. In addition, thefrictional stress created by the green compacts 95 being sintered canalso be reduced, thus minimizing the damage or deformation of theresultant sintered bodies.

[0057] However, depending on the shape, size or orientation direction ofthe green compacts 95, the sintered bodies 95 might be warped around thebottom 95 c thereof. For example, if the green compacts 95 are eitherrelatively tall or oriented in the height direction (i.e., vertically),then the green compacts 95 may shrink to a greater degree in the heightdirection. Or the bottom 95 c and surrounding portion of the greencompacts 95 may have their vertical cross-sectional shape deformed intoa trapezoidal shape due to their own weight. For example, when apressure of approximately 20 g/cm² or more is applied on the bottom 95 cof the green compacts 95, the green compacts 95 may be deformed there.In that situation, the green compacts 95 are crushed so to speak, andhave a broadened bottom 95 c. Nevertheless, if the green compacts 95 arearranged as is done in this preferred embodiment, just the bottom 95 cand surrounding portion of the green compacts 95 are deformed asdescribed above. Thus, by removing (e.g., cutting or grinding away) onlythose deformed portions, for example, the remaining portion of thesintered bodies 95 still can be used, thus increasing the yield of thematerial (or sintered bodies). When it is expected that it would bedifficult to avoid such deformation considering the selected shape ofthe green compacts 95, the green compacts 95 may be formed to have agreater size than required so as to easily cope with the deformation byremoving the unnecessary, deformed portions therefrom. In this manner,sintered bodies of a desired size can also be obtained.

[0058] In the example illustrated in FIGS. 1A, 1B and 2, the greencompacts 95 are arranged on the base plate 94 so as to have their bottom95 c contact with the base plate 94. Alternatively, depending on theshape of the green compacts 95, the green compacts 95 may also bearranged there so as to have their side surface 95 d contact with thebase plate 94. However, it is still most preferable to arrange the greencompacts 95 on the base plate 94 with their bottom 95 c facedown so thatthe projection area of each green compact 95 onto the base plate 94 isminimized.

[0059] Thus, according to various preferred embodiments of the presentinvention described above, sintered bodies can be obtained at a muchhigher yield and sintered magnets for use in a motor, for example, canbe produced much more efficiently. The method for producing rare earthsintered magnets according to preferred embodiments of the presentinvention can be used particularly effectively to prepare sinteredbodies in a shape that is very similar to that of the sintered magnetsto be obtained finally.

[0060] In the preferred embodiments described above, the green compactsto be processed into sintered magnets for use in a motor, for example,have inner and outer curved surfaces with mutually different radii ofcurvature. However, it is naturally possible to apply the presentinvention to green compacts having inner and outer curved surfaces withapproximately equal radii of curvature. Even in such an alternativepreferred embodiment, the area of contact between each green compact andthe receiving plane of the sintering case is also smaller than the areaof contact between two adjacent green compacts. Furthermore, the presentinvention is equally applicable to a thin-plate green compact, which hasa substantially rectangular parallelepiped shape (e.g., for an IMPmotor) and in which the powder is oriented in the thickness direction ofthe green compact.

[0061] In the preferred embodiments described above, the green compactsare arranged on the horizontal receiving plane of the sintering case sothat the bottom of those green compacts (i.e., a plane that is incontact with the receiving plane of the sintering case) and a plane onwhich the green compacts are in contact with each other (i.e., a sidesurface of the green compacts) cross at right angles. However, thepresent invention is not limited to those specific preferredembodiments. For example, where the bottom of the green compacts istilted, i.e., when the bottom of the green compacts and the plane onwhich the green compacts are in contact with each other (i.e., the sideface of the green compacts) do not cross at right angles, a sinteringcase, having a receiving plane that defines such an angle as to bringthe green compacts into contact with each other horizontally, may beused. For example, such a receiving plane may be the rugged surface of abase plate having a sawtooth cross section. Then, the green compacts canalso be arranged on the receiving plane stably.

[0062] A rare earth alloy powder for use in the method for producingrare earth sintered magnets according to preferred embodiments of thepresent invention is not particularly limited. For example, an R-T-(M)-Btype rare earth alloy powder as disclosed in U.S. Pat. No. 4,770,723 orNo. 4,792,368 may be used. An R-T-(M)-B type rare earth alloy powder,prepared by a strip casting process as disclosed in U.S. Pat. No.5,383,978, for example, is particularly preferred to achieve goodmagnetic properties. The contents of U.S. Pat. Nos. 4,770,723, 4,792,368and 5,383,978 identified above are hereby incorporated by reference. Thecompacting process may be performed by any of various known techniques.The green density is normally about 3.9 g/cm³ to about 5.0 g/cm³ and isoften about 4.1 g/cm³ to about 4.4 g/cm³.

[0063] To achieve sufficiently good magnetic properties andcompactability, the rare earth alloy powder for use to produce a rareearth sintered magnet according to a preferred embodiment of the presentinvention preferably has a mean particle size (i.e., FSSS particle size)of about 2 μm to about 10 μm, more preferably about 3 μm to about 6 μm.Also, the green density is preferably about 4.1 g/cm³ to about 4.5g/cm³. The reason is as follows. If the green density is lower thanabout 4.1 g/cm³, then the green compacts being sintered might bedeformed considerably. On the other hand, if the green density exceedsabout 4.5 g/cm³, then the magnetic powder will exhibit a decreaseddegree of orientation. The vertical length (i.e., the height) of thegreen compacts arranged on the base plate is preferably at most about 70mm. The arrangement adopted in the preferred embodiments of the presentinvention is particularly effective when the height is about 25 mm ormore.

[0064] After having been stored in the sintering case 9 in theabove-described manner, the green compacts 95 are sintered by heatingthe sintering case 9 in its entirety. The sintering process may also beperformed by a known technique and the conditions thereof may beoptimized in accordance with the type of the rare earth sintered magnetsto be produced. For example, the green compacts 95 may be sinteredthrough the following manufacturing and processing steps.

[0065] First, at least the sintering case 9 is loaded into a preparationchamber, which is provided at the inlet of a sintering apparatus, andthen the preparation chamber is sealed hermetically. Next, thepreparation chamber is evacuated to a pressure of about 2 Pa forantioxidizing purposes.

[0066] Then, the sintering case 9 is transported to a burn-off chamber,where the green compacts 95 are subjected to a binder removal processfor approximately 1 to 6 hours at a temperature of about 100° C. toabout 600° C. and at a pressure of about 2 Pa. The binder removalprocess is performed to volatilize and remove the lubricant (or binder),covering the surface of the magnetic powder, before the powder issintered. To improve the orientation of the magnetic powder during thecompaction process, the lubricant was mixed with the magnetic powderbefore the powder is pressed and compacted. The lubricant is presentbetween the particles of the magnetic powder.

[0067] After the binder removal process is finished, the sintering case9 is transported to a sintering chamber, where the green compacts 95 aresintered at about 1,000° C. to about 1,100° C. for approximately 2 to 5hours within a reduced pressure atmosphere (e.g., an Ar gas having apressure of about 2 Pa). Thereafter, the sintering case 9 is transportedto a cooling chamber, where the sintered bodies are cooled until thetemperature of the sintering case 9 reaches approximately roomtemperature.

[0068] Finally, the sintering case 9 is unloaded from the coolingchamber and then loaded into an aging treatment furnace, where thesintered bodies are subjected to a normal aging treatment. The agingtreatment may be conducted at a temperature of about 400° C. to about600° C. for approximately 1 to 5 hours within an inert atmosphere (e.g.,argon) at about 2 Pa.

INDUSTRIAL APPLICABILITY

[0069] Various preferred embodiments of the present invention describedabove provide a method for producing rare earth sintered magnets thatminimizes the number of damaged or deformed sintered bodies and greatlyincreases productivity. Also, even if any sintered bodies have beenpartially deformed, the deformed portions may be removed and theremaining portion still may be used, thus increasing the yield of thematerial advantageously. The method for producing rare earth sinteredmagnets according to preferred embodiments of the present invention canbe used particularly effectively to produce quasi-flat-plate sinteredmagnets having curved surfaces for use in a motor, for example.

[0070] It should be understood that the foregoing description is onlyillustrative of the present invention. Various alternatives andmodifications can be devised by those skilled in the art withoutdeparting from the present invention. Accordingly, the present inventionis intended to embrace all such alternatives, modifications andvariances which fall within the scope of the appended claims.

1. A method for producing rare earth sintered magnets, the method comprising the steps of: (a) pressing and compacting an alloy powder for the rare earth sintered magnets, thereby preparing a plurality of green compacts; (b) arranging the green compacts on a receiving plane in a direction in which a projection area of each of said green compacts onto the receiving plane is not maximized; and (c) heating the green compacts, thereby sintering the green compacts and obtaining a plurality of sintered bodies.
 2. The method of claim 1, wherein the step (b) includes the step of arranging the green compacts on the receiving plane in a direction in which the projection area of each of said green compacts onto the receiving plane is minimized.
 3. The method of claim 1 or 2, wherein the step (a) includes the step of preparing a plurality of green compacts each having at least one curved surface, and wherein the step (b) includes the step of arranging the green compacts on the receiving plane so that the at least one curved surface of each of said green compacts crosses the receiving plane substantially at right angles.
 4. The method of one of claims 1 to 3, wherein the step (a) includes the step of preparing a plurality of green compacts each having: two principal surfaces that are opposed to each other; two side surfaces that are opposed to each other with the two principal surfaces interposed therebetween; and two end surfaces that cross both the principal surfaces and the side surfaces substantially at right angles, and wherein the step (b) includes the step of arranging the green compacts on the receiving plane so that one of the two end surfaces of each of said green compacts contacts with the receiving plane.
 5. The method of one of claims 1 to 4, wherein the step (a) includes the step of pressing and compacting the alloy powder under an aligning magnetic field, and wherein the step (b) includes the step of arranging the green compacts on the receiving plane so that orientation directions of the alloy powder are substantially parallel to the receiving plane.
 6. The method of one of claims 1 to 5, wherein the step (a) includes the step of preparing the green compacts having a green density of about 4.1 g/cm³ to about 4.5 g/cm³.
 7. The method of one of claims 1 to 6, wherein the step (b) includes the step of arranging the green compacts on the receiving plane so that the green compacts are in contact with each other in a horizontal direction.
 8. The method of claim 7, wherein the step (b) includes the step of arranging the green compacts, which have already been magnetized, on the receiving plane so that the green compacts attract each other via a magnetic force produced between the green compacts.
 9. The method of claim 7 or 8, wherein the step (b) includes the step of applying an anti-fusing agent to at least portions of the green compacts and arranging the green compacts on the receiving plane so that the green compacts are in contact with each other via the anti-fusing agent.
 10. The method of claim 9, wherein the anti-fusing agent includes a powder of Y₂O₃.
 11. The method of claim 10, wherein the Y₂O₃ powder has a mean particle size of about 1 μm to about 10 μm.
 12. The method of claim 10 or 11, wherein the step (b) includes the step of applying slurry, in which the Y₂O₃ powder is dispersed in an organic solvent, to the portions of the green compacts.
 13. The method of one of claims 1 to 12, further comprising the step of removing a portion of each of said sintered bodies, which portion has been in contact with the receiving plane, and a surrounding portion thereof.
 14. A sintered magnet for use in a motor, wherein the magnet is produced by the method as recited in one of claims 1 to
 13. 